A conventional automobile transmission gear has been manufactured by carburizing and hardening a steel material after the steel material was gear cut. However, there was a problem of deformation of a member due to heat treatment strain.
By contrast, a spherical graphite cast iron can be readily manufactured. However, it has a disadvantage that it can not be used in an automobile transmission gear because of a low fatigue strength. Accordingly, it is desired for a cast iron material which was not carburized and not hardened so as to have a fatigue strength being the same as that of a carburized and hardened steel material.
A spherical graphite cast iron has a high mechanical strength in cast irons. As a technology for improving a fatigue strength of a spherical graphite cast iron, there is an austempering treatment applying to a spherical graphite cast iron containing, by weight ratio, 2.0 to 4.0% C, 1.5 to 4.5% Si, 2.0% or less Mn, 0.08% or less P, 0.03% or less S, 0.02 to 0.1% Mg, and 1.8 to 4.0% Cu.
The bending fatigue strength at 107 cycles of a spherical graphite cast iron having such the composition is shown in FIG. 13. As shown in a rotating bending test curve L of FIG. 13 where a stress (MPa) is shown in a vertical axis and the number of times of repetition of bending is shown in a horizontal axis, even a high tensile cast iron having a tensile strength such high as 1400 MPa only has a fatigue strength of about 200 MPa. This numerical value is comparable to that of a forged article, and the strength of 600 MPa or more being the same level as that of a carburized and hardened steel material is not obtained.
The fatigue strength of “about 200 MPa” can not be used in an automobile transmission gear.
As an another prior art, a technology is proposed, according to which a spherical graphite cast iron is cast to improve the fatigue strength thereof by means of adding an additive to a molten metal of a flake graphite cast iron (see Patent Document 1).
However, such the prior art intends to improve the fatigue strength by improving a casting step and can not improve the fatigue strength of a material after a cast iron material was mechanically machined.